1. Field of the Invention
The invention relates to an electroacoustic transducer with small dimensions, in particular, transducers having a housing with an outer diameter of 15 mm or less.
2. Description of the Related Art
With increasing miniaturization of electroacoustic transducers for use in cordless telephones, cellular telephones, head sets, dictation devices, computer monitors etc., there are problems which are unknown in classic transducer technology. An important problem is trying to maintain the transducer characteristics in view of the relative tolerances which, as a result of the reduction of the dimensions, become increasingly greater while at the same time trying to keep constant or even reduce the cost per piece, because, as a result of the high production numbers, the question of cost is of much greater significance than in the case of large-volume transducers which are produced in small production numbers for special applications.
Classic electroacoustic transducers which operate according to the electro-dynamic principle are comprised essentially of a cup-shaped housing which is open at the front and into which a magnetic system is introduced from the front. The magnetic system is comprised of the actual magnet and a multi-part yoke wherein the yoke forms at the front side an annular air gap. A diaphragm, which is fastened on the housing at the front side of the magnetic system, has a oscillation coil which projects into the air gap of the yoke and can oscillate in this air gap in the axial direction. The individual yoke parts are usually the following: a cup-shaped part arranged at the bottom, having the magnet glued thereto at its center, and a circular front part whose outer circumference forms the inner circumference of the air gap.
The special problem of the miniaturization is the centricity or eccentricity between the air gap and the oscillation coil because, when any eccentricity is present, the diaphragm begins to wobble and the transducer characteristics are significantly disturbed. In the case of miniaturization of the transducer, the eccentricities, which for large-size transducers are still permissible, present a serious quality problem because the relative eccentricity for the same absolute tolerances and miniaturization of the components increases, of course, with the degree of miniaturization. For example, in the case of transducers with a diameter of the housing of approximately 15 mm it is already required to lower the eccentricity into the range of less than hundredths of millimeters in order to achieve the predetermined transducer characteristics without distortion.
When it is also taken into consideration that the tolerances of the usually employed parts, i.e., housing, magnetic system, securing rings and the like, in the normal transducer construction are in the range of tenths of millimeters, while in the case of highly precise parts as they are mandatorily required for the small transducers the tolerances are in the range of five hundredths of millimeters, it is immediately clear that even for average pairs of tolerances eccentricities can occur easily which are no longer within acceptable limits. In the case of unfavorable pairs, the permissible eccentricity is easily surpassed.